I am getting slowly annoyed with comment spam. I don't get a lot, maybe one every few days. It's easy to delete, but that's not the point.
I have a solution. I could make it a "preference" that you are a "Human", thus able to make comments. Sure that would mean an extra click to the Preferences page (or even have it where the comments form was originally), but would be sticky (as long as you accepted the session cookie, or, as an alternative, set it each time you visited (as PHP's session tracking should then keep you going at least for that series of visits).
It seems like it might work. Would the spam bots work it out? Who knows. I don't know if any of them actually support cookies/sessions for posting comment spam into websites.
Something I shall look into, at some stage, when I have some time. Which may happen.
